The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having high energy density for use as a power source in these portable electronic instruments. Materials that are capable of reversible intercalation/deintercalation of lithium ions such as lithium-transition metal oxides are used as a positive active material of a lithium secondary battery, and lithium metals, lithium-containing alloys, or materials that are capable of reversible intercalation/deintercalation of lithium ions such as crystalline or amorphous carbons, or carbon-containing composites are used as a negative active material of a lithium secondary battery.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7V, which is higher than other alkali batteries, Ni—MH batteries, Ni-Cd batteries, etc. However, an electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2V is required in order to generate such a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, etc., is used as an electrolyte. However, such an electrolyte has significantly lower ion conductivity than an aqueous electrolyte that is used in a Ni—MH battery or a Ni-Cd battery, thereby resulting in the deterioration of battery characteristics during charging and discharging at a high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents the disintegration of the structure of the carbon negative electrode, which causes organic solvents in an electrolyte with a high molecular weight to make solvate lithium ions, and the solvent and the solvated lithium ions co-intercalate into the carbon negative electrode.
Once the organic SEI film is formed, lithium ions do not again react with the carbon electrode or other materials such that an amount of lithium ions is maintained. That is, carbon of the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode such that the electrolyte solution no longer decomposes, and stable charging and discharging are maintained (J. Power Sources, 51 (1994), 79–104). Because of these reasons, in the lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
In the case of a thin prismatic battery, a problem occurs in which gases are generated inside the battery due to a decomposition of a carbonate based organic solvent during the organic SEI film-forming reaction (J. Power Sources, 72(1998), 66–70). These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used. The thickness of the battery increases during charging due to the generation of gas inside the battery, and a passivation layer is slowly disintegrated by electrochemical energy and heat energy, which increase with the passage of time when the battery is stored at high temperatures after it is charged. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously. Furthermore, the internal pressure of the battery increases with this generation of gas. The increase in the internal pressure induces the deformation of the prismatic battery and lithium polymer battery. As a result, regional differences in the cohesion between pole plates inside an electrode element (positive and negative electrode, and separator) of the battery occur, thereby deteriorating the performance and safety of the battery and making it difficult to mount the lithium secondary battery set itself.
For solving the internal pressure problem, there is disclosed a method in which the safety of a secondary battery including a non-aqueous electrolyte is improved by mounting a vent or a current breaker for ejecting an internal electrolyte solution when the internal pressure is increased above a certain level. However, a problem with this method is that mis-operation may result from by an increase in internal pressure itself.
Furthermore, a method in which the SEI-forming reaction is changed by injecting additives into an electrolyte so as to inhibit the increase in internal pressure is known. For example, Japanese Patent Laid-open No. 97-73918 discloses a method in which high temperature storage characteristics of a battery are improved by adding 1% or less of a diphenyl picrylhydrazyl compound to the electrolyte. Japanese Patent Laid-open No. 96-321312 discloses a method in which cycle life and long-term storage characteristics are improved using 1 to 20% of an N-butyl amine group compound in an electrolyte. Japanese Patent Laid-open No. 96-64238 discloses a method in which storage characteristics of a battery are improved by adding 3×10−4 to 3×10−3 M of calcium salt to the electrolyte. Japanese Patent Laid-open No. 94-333596 discloses a method in which storage characteristics of a battery are improved by adding an azo compound to inhibit the reaction between an electrolyte and a negative electrode of the battery. In addition, Japanese Patent Laid-open No. 95-320779 discloses a method in which CO2 is added to an electrolyte, and Japanese Patent Laid-open No. 95-320779 discloses a method in which sulfide-based compounds are added to an electrolyte in order to prevent the electrolyte from decomposing.
Such methods as described above for inducing the formation of an appropriate film on a negative electrode surface such as an organic SEI film by adding a small amount of organic or inorganic materials are used in order to improve the storage characteristics and safety of a battery. However, there are various problems with these methods: the added compound is decomposed or forms an unstable film by interacting with the carbon negative electrode during the initial charge and discharge due to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in an electron; and gas is generated inside the battery such that there is an increase in internal pressure, resulting in significant worsening of the storage characteristics, safety, cycle life characteristics, and capacity characteristics of the battery.